Device and method for determining intermittent short circuit

ABSTRACT

A miniaturized cost-saving intermittent short circuit determining device located between a power source and an electric load in a vehicle electric circuit. The determining device includes detecting means for detecting current flowing through the electric circuit, determining means for determining whether the current detected by the detecting means is an intermittent short circuit, and disconnecting means for disconnecting the electric circuit when the determining means judges that there is an abnormality. The determining means is connected to an external switch circuit, which selectively supplies and stops current to the electric load of the electric circuit. The determining means determines whether the external switch circuit is switched on or off, and switches the disconnecting means on or off according to the determination.

BACKGROUND OF THE INVENTION

The present invention relates to a device and method for determining theoccurrence of an intermittent short circuit. More particularly, thepresent invention relates to an intermittent short circuit determiningdevice and an intermittent short circuit determining method that detectsovercurrent flowing through an electric circuit of an automobile.

U.S. Pat. No. 4,023,264 describes a blade fuse installed in a fuse boxof an automobile. Blade fuses, which are often used in electric circuitsof automobiles, have a slow-burn characteristic. Due to the slow-burncharacteristic, blade fuses are not melted by momentary excess currentsbut are melted by continuous excess currents that flow for a certainperiod of time.

Blade fuses normally melt and break when a dead short circuit occurs butdo not break when an intermittent short circuit occurs. A dead shortcircuit causes a large amount of current to continuously flow in anelectric circuit. An intermittent short circuit causes current to flowintermittently and within a short period of time in an electric circuit.An intermittent short circuit is a short circuit, which occurs when, forexample, vibrations cause the electric wiring of an automobile tocontact the body of the automobile. When an intermittent short circuitcurrent continuously flows through the electric circuit of theautomobile, for example, the electric circuit may be over-supplied withcurrent. Thus, there is a need for an intermittent short circuitdetermining device that accurately detects the occurrence of anintermittent short circuit.

One of the inventors of the present invention disclosed intermittentshort circuit determining devices in Japanese Unexamined PatentPublication Nos. 11-273544 (Japanese Patent Application No. 10-90866)and 2000-90808 (Japanese Patent Application No. 10-255204). Thedisclosed intermittent short circuit determining devices accuratelydetect the occurrence of intermittent short circuits. However, the priorart intermittent short circuit determining devices include separateparts for determining the occurrence of an intermittent short circuitand for disconnecting the electric circuit. This makes the prior artintermittent short circuit determining devices large and expensive.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a device and methodfor detecting the occurrence of an intermittent short circuit moreaccurately.

A further object of the present invention is to provide a compact andinexpensive intermittent short circuit determining device.

To achieve the above object, the present invention provides anintermittent short circuit determining device located between a powersource and a load circuit in a vehicle electric circuit. Theintermittent short circuit determining device includes detecting meansfor detecting current flowing through the load circuit, determiningmeans for determining whether the current detected by the detectingmeans is an intermittent short circuit, and disconnecting means fordisconnecting the load circuit when the determining means judges thatthere is an abnormality. The determining means is selectively connectedto an external switch circuit, which selectively supplies and stopscurrent to the load circuit. The determining means determines whetherthe external switch circuit is switched on or off, and switches thedisconnecting means on or off according to the determination.

The present invention also provides an intermittent short circuitdetermining device for determining whether an intermittent short circuithas occurred in a load circuit. The rare short circuit determiningdevice includes detecting means, determining means and disconnectingmeans. The detecting means is for detecting a load current, which flowsthrough the load circuit, and for generating a detection signal. Thedetermining means is connected to the sensor. The determining meansdetermines whether an intermittent short circuit has occurred based onthe detection signal. The disconnecting means is connected to thedetermining means. The disconnecting means stops the supply of the loadcurrent to the load circuit when it is determined that an intermittentshort circuit has occurred. The determining means determines whether anintermittent short circuit has occurred based on at least one of fourcharacteristic values. The characteristic values include the value of aload current flowing through the load circuit, a time period duringwhich the load current exceeds a current threshold value, an ON-dutyratio indicating the ratio of a time period when the load currentexceeds the current threshold value relative to a predetermined timeperiod, and an actual overcurrent number indicating the number of timesthat the load current exceeds the current threshold value during apredetermined time period. The determining circuit switches thedisconnecting means off according to the determination.

The present invention also provides a method for determining theoccurrence of an intermittent short circuit in a load circuit. Themethod includes at least one of four comparing steps including a currentcomparison step, a time period comparison step, a duty ratio comparisonstep, and an overcurrent number comparison step. The current comparisonstep compares the value of a load current flowing through the loadcircuit with a current threshold value. The time period comparing stepcompares a time period during which the load current exceeds the currentthreshold value with a reference time period. The duty ratio comparisonstep compares an ON-duty ratio with a reference ON-duty ratio. TheON-duty ratio indicates the ratio of a time period during which the loadcurrent exceeds the current threshold value relative to a predeterminedtime period. The overcurrent number comparison step compares an actualovercurrent number indicating the number of times that the load currentexceeds the current threshold value during the predetermined time periodwith a reference overcurrent number.

A further aspect of the present invention is a method for determiningwhether an intermittent short circuit has occurred in a load circuit.The method includes comparing the value of a load current that flows inthe load circuit with a first current threshold value, comparing theload current value with a second current threshold value, the secondcurrent threshold value being greater than the first current thresholdvalue, comparing a first time period, during which the load currentexceeds the first current threshold value, with a first reference timeperiod, comparing a second time period, during which the load currentexceeds the second current threshold value, with a second reference timeperiod, comparing an ON-duty ratio with a reference ON-duty ratio,wherein the ON-duty ratio is the ratio of a time period when the loadcurrent exceeds the second current threshold value relative to apredetermined time period, and comparing an overcurrent number with areference overcurrent number, wherein the overcurrent number is thenumber of times that the load current exceeds the second currentthreshold value during a predetermined time period.

Other aspects and advantages of the present invention will becomeapparent from the following description, taken in conjunction with theaccompanying drawings, illustrating by way of example the principles ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with objects and advantages thereof, may best beunderstood by reference to the following description of the presentlypreferred embodiments together with the accompanying drawings in which:

FIG. 1 is a schematic circuit diagram of an intermittent short circuitdetermining device according to a first embodiment of the presentinvention;

FIG. 2 is a schematic perspective view of the intermittent short circuitdetermining device of FIG. 1;

FIG. 3 is a schematic circuit diagram of an IC in the intermittent shortcircuit determining device of FIG. 1;

FIG. 4 is a flowchart illustrating an intermittent short circuitdetermining control program executed by a determining circuit of theintermittent short circuit determining device of FIG. 1;

FIG. 5 is a flowchart illustrating an intermittent short circuitdetermining control program executed by a determining circuit of theintermittent short circuit determining device of FIG. 1;

FIG. 6 is a flowchart illustrating an intermittent short circuitdetermining control program executed by a determining circuit of theintermittent short circuit determining device of FIG. 1;

FIG. 7 is a schematic circuit diagram of an intermittent short circuitdetermining device according to a second embodiment of the presentinvention;

FIG. 8A is a schematic plan view of the intermittent short circuitdetermining device of FIG. 7;

FIG. 8B is a schematic front view of the intermittent short circuitdetermining device of FIG. 7;

FIG. 8C is a schematic right side view of the intermittent short circuitdetermining device of FIG. 7;

FIG. 8D is a schematic bottom view of the intermittent short circuitdetermining device of FIG. 7;

FIG. 9 is a schematic right side view of the intermittent short circuitdetermining device of FIG. 7 in a moulded state;

FIG. 10A is a schematic perspective view illustrating a differentembodiment of the intermittent short circuit determining device of FIG.7;

FIG. 10B is a schematic perspective view illustrating a differentembodiment of the intermittent short circuit determining device of FIG.7;

FIG. 11A is a schematic perspective view illustrating a differentembodiment of the intermittent short circuit determining device of FIG.7;

FIG. 11B is a schematic perspective view illustrating a differentembodiment of the intermittent short circuit determining device of FIG.7;

FIG. 12 is a circuit diagram of an intermittent short circuitdetermining device according to a further embodiment of the presentinvention;

FIG. 13 is a circuit diagram of an intermittent short circuitdetermining device according to a further embodiment of the presentinvention; and

FIG. 14 is a circuit diagram of an intermittent short circuitdetermining device according to a further embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the drawings, like numerals are used for like elements throughout.

(First Embodiment)

FIG. 1 is a schematic circuit diagram of an intermittent short circuitdetermining device 80 according to a first embodiment of the presentinvention. FIG. 2 is a schematic perspective view of the intermittentshort circuit determining device 80. The intermittent short circuitdetermining device 80 is embodied in an automobile intermittent shortcircuit determining device. The intermittent short circuit determiningdevice 80 is connected to a terminal block (not shown), which isincluded in an electric circuit of the automobile.

As shown in FIG. 2, the intermittent short circuit determining device 80includes a conductive terminal 11, a signal input terminal 31, a currentsensor (detecting means) 2, a relay circuit (disconnecting means) 14, anoutput terminal 32, a determining circuit (determining means) 16, and arelay electrode 18. The relay circuit 14 includes a power MOSFET(hereinafter, simply referred to as FET).

The intermittent short circuit determining device 80 includes a moldedmaterial such as a molded package 17, which is made of epoxy resin. Theintermittent short circuit determining device 80 is formed into a flathexahedron. The conductive terminal 11 is flat and located on one sideof the package 17. An insulation (not shown), is formed on the innersurface of the conductive terminal 11. The determining circuit 16 islocated on the inner surface of the conductive terminal 11. Theinsulation is not formed on the outer surface (bottom surface in FIG. 2)of the conductive terminal 11.

The conductive terminal 11 includes an L-shaped branching arm 11 a. Therelay electrode 18 is located inside the branching arm 11 a, as shown inFIG. 2. The branching arm 11 a is connected to the relay electrode 18 bythe current sensor (thin fuse) 2. The current sensor 2 has apredetermined impedance Z. The current sensor 2 has a current capacitysuch that the current sensor 2 does not break when a current greaterthan or equal to a predetermined reference value (overcurrent)constantly flows for a predetermined reference time period.

The output terminal 32 is bent in an L-shaped manner outside the package17 as shown. The FET 14 is located on the flat surface of the outputterminal 32 inside the package 17. The drain of the FET 14 is connectedto the relay electrode 18 by a conductive bar 19. The source of the FET14 is connected to the output terminal 32. The gate of the FET 14 isconnected to a relay circuit control terminal 20 of the determiningcircuit 16 (see FIG. 1).

Input terminals 22, 23 of the determining circuit 16, which is shown inFIG. 1, are connected to the conductive terminal 11 and the relayelectrode 18 by lead wires 24, 25, respectively, as shown in FIG. 2. Theconductive terminal 11 and the relay electrode 18 are connected to theends of the current sensor 2, respectively. The determining circuit 16constantly receives the detection signal (voltage) through theconductive terminal 11 and the input terminals 22, 23.

A ground terminal 27 is bent in an L-shaped manner outside the package17. The end of the ground terminal 27 inside the package 17 is connectedto the determining circuit 16 by a lead wire 26.

If one of the determining circuit 16 and the FET 14 in the intermittentshort circuit determining device 80 does not function properly and adead short circuit occurs, the current sensor 2, which functions as afuse, breaks and the current supply to a load circuit 29 is stopped.

As shown in FIG. 2, the signal input terminal 31 is bent in an L-shapedmanner outside the package 17. The end of the signal input terminal 31in the package 17 is connected to an external signal input terminal 28of the determining circuit 16.

The signal input terminal 31 is connected to an external switch 30 ofthe automobile. When the external switch 30 is turned on, the signalinput terminal 31 receives a switch ON signal (current control signal).When the external switch 30 is turned off, the signal input terminal 31receives a switch OFF signal (current control signal).

The determining circuit 16 includes an IC 16A and a microcomputer 16B.The determining circuit 16 determines whether an intermittent shortcircuit has occurred. That is, the IC 16A receives the voltage on eachterminal of the current sensor 2 (potential difference) through theinput terminals 22, 23. The determining circuit 16 determines whetherthe received potential difference (load current) is greater than orequal to the predetermined threshold value by comparing the receivedvoltages. The determining circuit 16 provides the result to themicrocomputer 16B.

The microcomputer 16B includes a CPU 6, a ROM, and a RAM. Theintermittent short circuit determining program and a close/open controlprogram are stored in the ROM 7. The operation memory is stored in theRAM 8. The microcomputer 16B determines whether an intermittent shortcircuit has occurred based on the intermittent short circuit determiningprogram.

The battery BT of the automobile is connected to a load circuit 29 viathe current sensor 2 and a power MOSFET 14, or relay circuit. The loadcircuit 29 includes, for example, a headlamp or a radio. Further, theload circuit 29 includes the electric wiring (electric lines) connectedto the headlamp or the radio.

The determining circuit 16 of the intermittent short circuit determiningdevice 80 will now be discussed. FIG. 3 is a schematic circuit diagramof the IC 16A of the determining circuit 16.

The IC 16A includes a differential amplifier circuit 21, a firstcomparator circuit 12, a second comparator circuit 13, resistors R1, R2,R3, and a charge pump 4. The IC 16A is connected to the microcomputer16B via input signal lines IS1, IS2 and output signal lines OS1, OS2.

A non-inverting input terminal (hereinafter, referred to as positiveterminal) 21 a of the differential amplifier circuit (operationamplifier) 21 is connected to the input terminal 22. A potential E1 atthe battery side of the currnt sensor 2 is supplied to the positiveterminal 21 a of the differential amplifier circuit 21.

An inverting input terminal (hereinafter, referred to as negativeterminal) 21 b of the differential amplifier circuit 21 is connected tothe input terminal 23. A potential E2 at the load side of the currntsensor 2 is supplied to the negative terminal 21 b of the differentialamplifier circuit 21.

When the differential amplifier circuit 21 receives both potentials E1,E2 from the currnt sensor 2 as detection signals, the potentialdifference (E1−E2) is amplified by a predetermined factor. The potentialdifference (E1−E2) is amplified by N (N>0) times in the firstembodiment. This generates an amplified voltage Va (Va=N×(E1−E2)). Theamplified voltage Va is supplied from the output terminal 21 c to anegative terminal 12 a of the first comparator circuit (operationamplifier) 12 and to a negative terminal 13 a of the second comparatorcircuit (operation amplifier) 13.

The potential difference (E1−E2) is produced between the two terminalsof the currnt sensor 2 when the load current (detected current) IL thatflows through the load circuit 29 and the currnt sensor 2 has a currentvalue Id. The potential difference is described by “impedance Z×loadcurrent value Id”. Therefore, the amplified voltage Va is represented bythe following equation.Va=N×(E1−E2)=N×Z×Id

The amplified voltage Va is proportional to the current value Id, andthe impedance Z is substantially constant. Thus, the current value Idrepresents the amplified voltage Va.

The output terminal 21 c of the differential amplifier circuit 21 isconnected to the negative terminal 12 a of the first comparator circuit12. A non-inverting input terminal, or a positive terminal 12 b, of thefirst comparator circuit 12 is connected to a median point between theresistors R2 and R3. A first voltage threshold value VT1, which isrepresented by the following equation, is applied to the positiveterminal 12 b. The voltage value of the battery power source BT isrepresented by VB, the resistance value of the resistor R1 isrepresented by RA, the resistance value of the resistor R2 isrepresented by RB, and the resistance value of the resistor R3 isrepresented by RC.VT1=(VB×RC/(RA+RB+RC))

An output terminal 12 c of the first comparator circuit 12 is connectedto the microcomputer 16B via the output signal line OS1.

In the first embodiment, the first voltage threshold value VT1 is setsuch that it is equal to the amplified voltage Va when the load currentIL, which is equal to a first current threshold value A1, flows to theload circuit 29. The first current threshold value A1 is within thecurrent capacity range of the current supply lines.

The first comparator circuit 12 compares the amplified voltage Va withthe first voltage threshold value VT1. That is, the first comparatorcircuit 12 determines whether the current value Id is greater than thefirst current threshold value A1.

When the current value Id (amplified voltage Va) is less than or equalto the first current threshold value A1 (first voltage threshold valueVT1), the first comparator circuit 12 provides the microcomputer 16Bwith a first comparison signal having a high level. When the loadcurrent value Id is greater than the first current threshold value A1,the first comparator circuit 12 provides the microcomputer 16B with thefirst comparison signal at a low level.

The output terminal 21 c of the differential amplifier circuit 21 isconnected to the negative terminal 13 a of the second comparator circuit(operation amplifier) 13. A positive terminal 13 b of the secondcomparator circuit 13 is connected to a median point between theresistors R1 and R2. That is, a second voltage threshold value VT2,which is represented by the following equation, is applied to thepositive terminal 13 b.VT2=(VB×(RB+RC)/(RA+RB+RC))

An output terminal 13 c of the second comparator circuit 13 is connectedto the microcomputer 16B via the output signal line OS2.

In the first embodiment, the second voltage threshold value VT2 is setsuch that it is equal to the amplified voltage Va when the load currentIL, which is equal to a second current threshold value A2, flows to theload circuit 29.

After receiving the amplified voltage Va from the differential amplifiercircuit 21, the second comparator circuit 13 compares the amplifiedvoltage Va with the second voltage threshold value VT2. The secondcomparator circuit 13 determines whether the load current value Id isgreater than the second current threshold value A2. When the loadcurrent value Id (amplified voltage Va) is less than or equal to thesecond current threshold value A2 (second voltage threshold value VT2),the second comparator circuit 13 provides the microcomputer 16B with asecond comparison signal having a high level. When the load currentvalue Id is greater than the second current threshold value A2, thesecond comparator circuit 13 supplies the microcomputer 16B with thesecond comparison signal at a low level. The microcomputer 16Bdetermines whether the load current value Id is greater than thereference value (A1, A2).

In the first embodiment, the microcomputer 16B determines whether anintermittent short circuit has occurred based on the following fourcharacteristic values:

-   -   1. A first time period TA during which the load current IL        exceeds the first current threshold value A1;    -   2. A second time period TB during which the load current IL        exceeds the second current threshold value A2;    -   3. Ratio DU of the sum of second time period TB to a        predetermined time period (ON-duty ratio, hereinafter, referred        to as DUTY ratio); and    -   4. An overcurrent number KS indicating the number of times that        the load current IL exceeds a second current threshold value A2        during a predetermined time period.

An output terminal of the IC 16A is connected to the first outputterminal 20 of the determining circuit 16. The first output terminal 20is connected to the gate of the FET 14. When the IC 16A receives acurrent flow permission signal (first mode signal) from themicrocomputer (processing circuit) 16B, the IC 16A charges a charge pump4 and generates a FET ON signal that activates the FET 14. The FET ONsignal is sent to the gate of the FET 14 via the first output terminal20. When the FET ON signal is sent to the gate, the portion between thedrain and source of the FET 14 are connected. This supplies the loadcircuit 29 with the load current IL.

When the IC 16A receives a current shutdown signal (second mode signal)from the microcomputer 16B, the IC 16A discharges the charge pump 4 andgenerates a FET OFF signal, which inactivates the FET 14. When the FETOFF signal is provided to the gate, the drain and the source aredisconnected. This stops the supply of the load current IL to the loadcircuit 29.

Another output terminal (not shown) of the IC 16A is connected to alight-emitting diode (hereinafter, referred to as LED) on theautomobile. When the IC 16A receives a LED ON signal from themicrocomputer 16B, a LED goes on. When the IC 16A receives a LED OFFsignal from the microcomputer 16B, the LED goes off. The LED is, forexample, installed in an instrument panel of the automobile (not shown).

The operation of the intermittent short circuit determining device 80will now be described. FIGS. 4, 5, and 6 are flowcharts illustrating theprocess of the intermittent short circuit determining program. Theprogram is executed by the CPU 6 of the determining circuit 16.

The CPU 6 determines whether an intermittent short circuit has occurredin the electric circuit based on the intermittent short circuitdetermining program. When it is determined that an intermittent shortcircuit has not occurred, current is supplied to the load circuit 29.When it is determined that an intermittent short circuit has occurred,the supply of current to the load circuit 29 is stopped.

When the microcomputer 16B is supplied with a battery voltage VB fromthe battery power source BT, the CPU 6 resets counters CTA, CTB, C3, C4,C5, and C6 to zero in step S1. Then, the CPU 6 proceeds to step S2.

In step S2, the CPU 6 determines whether the switch ON signal has beenprovided. If it is determined that the switch ON signal has not beenreceived, the CPU 6 proceeds to step S16. If it is determined that theswitch ON signal has been received, the CPU 6 proceeds to step S3.

In step S16, the CPU 6 supplies the IC 16A with the current shutdownsignal and the LED OFF signal. When the IC 16A receives the currentshutdown signal, the IC 16A generates the FET OFF signal. The IC 16Ainactivates the FET 14 based on the FET OFF signal. When the IC 16Areceives the LED OFF signal, the IC 16A turns off the LED accordingly.The CPU 6 then returns to step S2.

In step S3, the CPU 6 provides the IC 16A with the current flowpermission signal. Upon receipt of the current flow permission signal,the IC 16A generates the FET ON signal, which activates the FET 14accordingly.

In step S4, the CPU 6 receives the load current value Id and proceeds tostep S5. In step S5, the CPU 6 determines whether the current value Idis greater than the first current threshold value A1 based on the firstand the second comparison signals.

When the first and second comparison signals are high, the CPU 6determines that the current value Id is less than or equal to the firstcurrent threshold value A1. When the first comparison signal is low andthe second comparison signal is high, the CPU 6 determines that thecurrent value Id is greater than the first current threshold value A1.

If it is determined that the current value Id is greater than the firstcurrent threshold value A1, the CPU 6 proceeds to step S6. If it isdetermined that the current value Id is less than or equal to the firstcurrent threshold value A1, the CPU 6 proceeds to step S17.

In step S17, the CPU 6 supplies the IC 16A with the current flowpermission signal and the LED OFF signal. Upon receipt of the currentflow permission signal, the IC 16A generates the FET ON signal, whichactivates the FET 14 accordingly. This allows the current to be suppliedto the load circuit 29. Upon receipt of the LED OFF signal, the IC 16Aturns off the LED. The microcomputer 16B then returns to step S2.

In step S6, the CPU 6 increments the counter CTA by a value of one andproceeds to step S7. The counter CTA calculates the first time period TAduring which the continuous load current IL exceeds the first currentthreshold value A1.

In step S7, the CPU 6 determines whether the current value Id is greaterthan the second current threshold value A2 according to the first andthe second comparison signals.

When the first and the second comparison signals are low, the CPU 6determines that the current value Id is greater than the second currentthreshold value A2 and proceeds to step S8. When the first comparisonsignal is low and the second comparison signal is high, the CPU 6determines that the current value Id is less than or equal to the secondcurrent threshold value A2 and proceeds to step S8.

If it is determined that the current value Id is greater than the secondcurrent threshold value A2, the CPU 6 proceeds to step S8. If it isdetermined that the current value Id is less than or equal to the secondcurrent threshold value A2, the CPU 6 proceeds to step S18.

In step S18, the CPU 6 determines whether the first time period TA isgreater than the first reference time period T1. If it is determinedthat the first time period TA is greater than the first reference timeperiod T1, the CPU 6 determines that the dead short may occur andproceeds to step S19. If it is determined that the first time period TAis less than or equal to the first reference time period T1, the CPU 6proceeds to step S17.

In step S19, CPU 6 supplies the IC 16A with the current shutdown signaland the LED ON signal. Upon receipt of the current shutdown signal, theIC 16A generates the FET OFF signal, which inactivates the FET 14accordingly. When the FET 14 is inactivated, the current supply to theload circuit 29 is stopped. This prevents the dead short circuit fromoccurring and protects the load circuit 29 against the overcurrentcaused by the dead short circuit. Upon receipt of the LED ON signal, theIC 16A turns on the LED. The CPU 6 proceeds to step S20.

In step S20, the CPU 6 send a DIAG signal and executes a self-diagnostictest. The CPU 6 then proceeds to step S21. In step S21, the CPU 6determines whether an ignition signal of the automobile has beenreceived. If it is determined that the ignition signal has not beenreceived, the CPU 6 proceeds to step S19. If it is determined that theignition signal is received, the CPU 6 proceeds to step S1.

If it is determined in step S7 of FIG. 4 that the load current value Idis greater than the second current threshold value A2, the CPU 6increments the count value of the counter CTB in step S8. The counterCTB calculates a second time period TB during which the continuouscurrent IL exceeds the second current threshold value A2. The CPU 6 thenproceeds to step S9.

In step S9, the CPU 6 increments the count value of an ON time periodcounter C3 and proceeds to step S10. The ON time period counter C3 addsup the ON time of the load current IL.

In step S10, the CPU 6 increments each count value of an overcurrentcounter C5 and a time period counter C6, and proceeds to step S11. Theovercurrent counter C5 calculates the overcurrent number, by which theload current IL exceeds the second current threshold value A2.

In step S11, the CPU 6 determines whether the second time period TB isgreater than a second reference time period T2 (T2<T1). If it isdetermined that the second time period TB is greater than the secondreference time period T2, the CPU 6 determines that an intermittentshort circuit has occurred and proceeds to step S19. In step S19, theCPU 6 provides the IC 16A with the current shutdown signal andinactivates the FET 14.

In step S11, if it is determined that the second time period TB is lessthan or equal to the second reference time period T2, the CPU 6 proceedsto step S12. In step S12, the microcomputer 16B calculates the ratio DU(duty ratio) of the ON time period of the load current IL to thepredetermined time period. The microcomputer 16B calculates the ratio DUbased on the value of the ON time period counter C3.

In step S13, CPU 6 determines whether the duty ratio DU is greater thana reference duty ratio D1. If it is determined that the duty ratio DU isgreater than the reference duty ratio D1, the CPU 6 determines that anintermittent short circuit has occurred and proceeds to step S19. Instep S19, the CPU 6 provides the IC 16A with the current shutdown signaland inactivates the FET 14. This protects the load circuit 29. If it isdetermined that the duty ratio DU is less than or equal to the referenceduty ratio D1, the CPU 6 proceeds to step S14.

In step S14, the CPU 6 calculates the overcurrent number KS, which isthe number of times the load current IL exceeds the second currentthreshold value A2 during the predetermined time period. The CPU 6calculates the overcurrent number KS based on the value of theovercurrent counter C5 and the value of the time period counter C6.

In step S15, the CPU 6 determines whether the overcurrent number KS isgreater than the reference overcurrent number K1. If it is determinedthat the overcurrent number KS is greater than a reference overcurrentnumber K1, the CPU 6 determines that an intermittent short circuit hasoccurred and proceeds to step S19. In step S19, the CPU 6 supplies theIC 16A with the current shutdown signal and inactivates the FET 14. Thisprotects the load circuit 29. If it is determined that the overcurrentnumber KS is less than or equal to the reference overcurrent number K1,the CPU 6 proceeds to step S17.

The first and second reference time periods T1, T2, the reference dutyratio D1, and the reference overcurrent number K1 are stored in the ROM7.

The microcomputer 16B receives the switch ON signal when the externalswitch 30 is switched on. The microcomputer 16B closes the FET 14 basedon the close/open control program. The microcomputer 16B provides thesignal to the gate of the FET 14 via the relay circuit control terminal20 of the IC 16A to activate the FET 14.

The microcomputer 16B receives the switch OFF signal when the externalswitch 30 is turned off. The microcomputer 16B opens the FET 14 based onthe close/open control program. The microcomputer 16B provides thesignal to the gate of the FET 14 via the relay circuit control terminal20 of the IC 16A to inactivate the FET 14.

The intermittent short circuit determining device 80 according to thefirst embodiment provides the following advantages.

(1) The CPU 6 inactivates the FET 14 and stops the supply of current tothe load circuit 29 when it is determined that the following conditionsare met. The load current value Id is greater than the first currentthreshold value A1 and is less than or equal to the second currentthreshold value A2. The first time period TA is greater than the firstreference time period T1. This prevents a dead short circuit fromoccurring and protects the load circuit 29 of the automobile.

(2) The CPU 6 determines that a dead short circuit has occurred when thefollowing conditions are met. The load current value Id is greater thanthe second current threshold value A2. The second time period TB isgreater than the second reference time period T2. Then, the CPU 6inactivates the FET 14 and stops the supply of current to the loadcircuit 29. This protects the electric lines from overcurrent caused bya dead short circuit.

(3) The CPU 6 determines that an intermittent short circuit has occurredwhen the following conditions are met. The load current value Id isgreater than the second current threshold value A2. The second timeperiod TB is less than or equal to the second reference time period T2.The duty ratio DU of the detected current IL is greater than thereference duty ratio D1. Then, the CPU 6 inactivates the FET 14 andstops the supply of current to the load circuit 29. This protects theelectric lines from overcurrent caused by an intermittent short circuit.

(4) The CPU 6 determines that an intermittent short circuit has occurredwhen the following conditions are met: the load current value Id isgreater than the second current threshold value A2; the second timeperiod TB is less than or equal to the second reference time period T2;the duty ratio DU is less than or equal to the reference value D1; andthe overcurrent number KS is greater than the reference overcurrentnumber K1. Then, the CPU 6 inactivates the FET 14 and stops the supplyof current to the load circuit 29. This protects the electric lines fromovercurrent caused by the intermittent short circuit.

(5) The current sensor 2 functions as a fuse. Therefore, even if thedetermining circuit 16 or the FET 14 fails to function because of, forexample, the occurrence of a dead short circuit in the load circuit 29,the current sensor 2 breaks as a fuse and stops the supply of current tothe load circuit 29.

(6) The determining circuit 16 of the intermittent short circuitdetermining device 80 is connected to the external switch 30 (externalswitch circuit). The external switch 30 determines whether or not tosupply the load current IL to the load circuit 29. When it is determinedto switch the external switch 30 on, the determining circuit 16activates the FET 14. When it is determined to switch the externalswitch 30 off, the determining circuit 16 inactivates the FET 14.Therefore, the current supply to the load circuit 29 is controlled bythe operation of the external switch 30.

(7) The intermittent short circuit determining device 80 includes thecurrnt sensor 2, the FET 14, and the determining circuit 16, which areintegrated in the molded package 17. Thus, the size of the intermittentshort circuit determining device 80 is reduced and it is inexpensive.

(8) The relay circuit includes the FET 14. As a result, theconfiguration of the relay circuit is simplified and the size of theintermittent short circuit determining device 80 is reduced. The sizereduction is also advantageous for packaging the device 80.

The first embodiment of the present invention may be changed as follows.

In step S5 of FIG. 4, if it is determined that the load current value Idis greater than the first current threshold value A1, the CPU 6 mayproceed to step S18. In step S18, if it is determined that the firsttime period TA is less than or equal to the first reference time periodT1, the CPU 6 may proceed to step S7.

In step S5 of FIG. 4, if it is determined that the load current value Idis greater than the first current threshold value A1, the CPU 6 mayproceed to step S19. In this case, the steps S6, S7, S8, S9, S10, S11,S12, S13, S14, S15, and S18 are omitted.

The IC 16A may have only one comparator circuit. The IC 16A may alsohave more than three comparator circuits.

The first mode signal and the second mode signal may be formed of onlythe current flow permission signal and the current shutdown signal,respectively.

The current threshold value, the reference duty ratio, and the referenceovercurrent number K1 are set as required. That is, different currentthreshold values may be set as the load current Id. Load current rangesare defined by the current threshold values. At least one of thereference ON duty ratio and the reference overcurrent number K1 may beset in correspondence with each load current range. In this case, theintermittent short circuit determining program is executed for each loadcurrent range.

The following tables 1, 2 and 3 show the examples of combinations ofconditions for the CPU 6 to determine that an intermittent short circuithas occurred. The occurrence of an intermittent short circuit may bedetermined based on the combinations listed in the tables 1 to 3. Eachcombination of circles in a column indicates abnormal conditions.

Table 1 shows load current ranges defined by the current thresholdvalues A1, A2, A3, A4, A5 (A1<A2<A3<A4<A5) and duty ratio areas definedby the duty ratios D1, D2, D3, D4, D5 (D1<D2<D3<D4<D5). The row marked“A1≦” indicates that A1≦load current<A2. The circles in the lowest rowof the current value and in the lowest row of the duty ratio indicatethat each of the detected value is greater than or equal to theindicated value of the associated row. For example, the circle in therow marked “A5≦” represents that the detected value is greater than orequal to A5. For example, “D1≦duty ratio<D2” corresponds to “A1≦loadcurrent<A2”. When the combination is “A1≦load current<A2” and “D1≦dutyratio<D2”, it is determined that an intermittent short circuit hasoccurred.

TABLE 1 Intermittent short Circuit Comb. 1 Comb. 2 Comb. 3 Comb. 4 Comb.5 Current Value A1 ≦ ◯ A2 ≦ ◯ A3 ≦ ◯ A4 ≦ ◯ A5 ≦ ◯ Duty Ratio D1 ≦ ◯ D2≦ ◯ D3 ≦ ◯ D4 ≦ ◯ D5 ≦ ◯

Table 2 shows the load current ranges defined by the current values A1to A5 and over current number ranges defined by the numbers K1, K2, K3,K4, K5 (K1<K2<K3<K4<K5). “K1≦overcurrent number<K2” corresponds to“A1≦load current<A2”. For, example when A1≦load current<A2 andK1≦overcurrent number<K2, it is determined that an intermittent shortcircuit has occurred.

TABLE 2 Intermittent short Circuit Comb. 6 Comb. 7 Comb. 8 Comb. 9 Comb.10 Current Value A1 ≦ ◯ A2 ≦ ◯ A3 ≦ ◯ A4 ≦ ◯ A5 ≦ ◯ Overcurrent NumberK1 ≦ ◯ K2 ≦ ◯ K3 ≦ ◯ K4 ≦ ◯ K5 ≦ ◯

Table 3 shows the load current ranges defined by the current values A1to A5, the duty ratio ranges defined by the duty ratios D6, D7, D8, D9,D10 (D5<D6<D7<D8<D9<D10), and the overcurrent number ranges defined bythe overcurrent number K6, K7, K8, K9, K10 (K5<K6<K7<K8<K9<K10). Forexample, “D6≦duty ratio<D7” and “K6≦overcurrent number<K7” correspond to“A1≦load current A2”. For example, when the combination is A1≦loadcurrent<A2 and D6≦duty ratio<D7 or K1≦overcurrent number<K2, it isdetermined that an intermittent short circuit has occurred.

TABLE 3 Intermittent short Circuit Comb. 11 Comb. 12 Comb. 13 Comb. 14Comb. 15 Current Value A1 ≦ ◯ A2 ≦ ◯ A3 ≦ ◯ A4 ≦ ◯ A5 ≦ ◯ Duty Ratio D6≦ ◯ D7 ≦ ◯ D8 ≦ ◯ D9 ≦ ◯ D10 ≦ ◯ Overcurrent Number K6 ≦ ◯ K7 ≦ ◯ K8 ≦ ◯K9 ≦ ◯ K10 ≦ ◯

Tables 1 to 3 are stored in the ROM 7 as maps. The CPU 6 refers to thereference value of each load current range when determining theoccurrence of the intermittent short circuit.

(Second Embodiment)

FIG. 7 is a schematic circuit diagram of an intermittent short circuitdetermining device 90 according to a second embodiment of the presentinvention. The intermittent short circuit determining device 90 isembodied in an automobile intermittent short circuit determining device.

FIG. 8A is a plan view of the intermittent short circuit determiningdevice 90 without molded material. FIG. 8B is a front view of thedetermining device 90. FIG. 8C is a right side view of the determiningdevice 90. FIG. 9 is a right side view of the intermittent short circuitdetermining device 90.

In the second embodiment, as shown in FIG. 8D, a rectangular multi-layerinsulating substrate 41 has a wide conductive pattern (hereinafter,referred to as the conductive terminal pattern) 42 a. The conductiveterminal pattern 42 a has three branching conductors 43. The branchingconductors 43 are connected to lands 44 (see FIG. 8A) through platedthrough holes (not shown), respectively. The lands 44 and the platedthrough holes are located on the multi-layer insulating substrate 41(upper surface in FIG. 8B).

One end of each thin first, second, and third current sensor 45 a, 45 b,and 45 c, the characteristics of which depend on the current capacity,is connected to one of the lands 44. Each first, second, and thirdcurrent sensor 45 a, 45 b, and 45 c, transverses one of the recesses 46on the multi-layer insulating substrate 41. Another end of each first,second, and third current sensor 45 a, 45 b, and 45 c, is connected toone of the other lands 47 located on the multi-layer insulatingsubstrate 41.

The lands 47 are connected to the drains (not shown) of the first,second and third FET (relay circuit) 49 a, 49 b, and 49 c by theconnecting leads 48, respectively. The current capacity of eachconnecting lead 48 is greater than that of the corresponding first,second, and third current sensor 45 a, 45 b, and 45 c.

The first, second, and third FETs 49 a, 49 b, 49 c are each fixed to oneend of each first, second, and third conductive terminal 50 a, 50 b, and50 c via the associated sources (not shown). The conductive terminals 50a, 50 b, and 50 c are located on the multi-layer insulating substrate41. The other end of each of the first, second, and third conductiveterminals ⁵ 0 a, 50 b, 50 c projects outward from the multi-layerinsulating substrate 41.

The determining circuit 51 is fixed to a GND connecting terminal 55formed on the multi-layer insulating substrate 41. The internal circuitof the determining circuit 51 is grounded via the GND connectingterminal 55. One end of the GND connecting terminal 55 projects outwardfrom the multi-layer insulating substrate 41.

Each land 44 is connected to one end of each first, second, and thirdcurrent sensor detect terminals (not shown) of the determining circuit51 via plated through holes (not shown), wiring patterns located on theinternal layer of the multi-layer insulating substrate 41, and leadwires.

Each land 47 is connected to the other end of each first, second, andthird current sensor detect terminals (not shown) of the determiningcircuit 51 via the plated through holes (not shown), wiring patternslocated on the internal layer of the multi-layer insulating substrate41, and lead wires.

The first, second, and third output terminal of the determining circuit51 is connected to the gates (not shown) of the first, second, and thirdFET 49 a, 49 b, 49 c via lead wires and wiring patterns located on themulti-layer insulating substrate 41, respectively. A power source inputterminal (not shown) for the internal circuit, which is located on theback surface of the determining circuit 51, is connected to theconductive terminal pattern 42 a via the plated through hole.

Three external signal input terminals of the determining circuit 51 areconnected to the first, second, and third signal input terminals 52 a,52 b, 52 c on the multi-layer insulating substrate 41 via lead wires,respectively. The other end of each first, second, and third signalinput terminal 52 a, 52 b, 52 c projects outward from the multi-layerinsulating substrate 41.

The first, second, and third signal input terminals 52 a, 52 b, and 52 care connected to the first, second, and third automobile externalswitches 30 a, 30 b, and 30 c (see FIG. 7), respectively. The automobileexternal switches function as the external switch circuit of theautomobile. When any of the first, second, and third external switches30 a, 30 b, and 30 c is switched on, a switch ON signal is provided tothe determining circuit 51. When any of the first, second, and thirdexternal switches 30 a, 30 b, and 30 c is switched off, a switch OFFsignal is provided to the determining circuit 51.

As shown in FIG. 9, each member arranged on the multi-layer insulatingsubstrate 41 is packaged with, for example, the molded material, whichis made of epoxy resin. The intermittent short circuit determiningdevice 90 is formed into a flat hexahedron. The first, second, and thirdconductive terminals 50 a, 50 b, 50 c, the first, second, and thirdsignal input terminals 52 a, 52 b, 52 c, and one end of the GNDconnecting terminal 55 project outward from the package.

The determining circuit 51 includes an IC and a microcomputer as in thedetermining circuit 16 of the second embodiment. The determining circuit51 determines the occurrence of an intermittent short circuit withrespect to the first, second, and third current sensors 45 a, 45 b, and45 c. The determining circuit 51 selectively closes and opens each FET49 a, 49 b, and 49 c based on the switch ON/OFF signal of thecorresponding first, second, and third external switch 30 a, 30 b, and30 c. The intermittent short circuit determining method and theclose/open control of FET 49 a, 49 b, 49 c are equivalent to those ofthe first embodiment.

The intermittent short circuit determining device 90 is arranged on theterminal block (not shown), which is provided in the electric circuit ofautomobiles. The first, second, or third current sensor 45 a, 45 b, 45 cbreaks and stops the supply of current if the determining circuit 51 ofthe intermittent short circuit determining device 90 or one of the FET49 a, 49 b, 49 c fails to operate properly. The failure may occur due tothe occurrence of a dead short circuit in one of the load circuits 54 a,54 b, 54 c. A dead short circuit causes a large amount of current tocontinuously flow in an electric circuit.

The load circuits 54 a, 54 b, 54 c include, for example, a lamp(headlamp, fog lamp), a radio, or car audio equipment.

The intermittent short circuit determining device 90 of the secondembodiment provides the following advantages.

(1) The first, second, and third current sensors 45 a, 45 b, 45 c, andthe first, second, and third FET (relay circuit) 49 a, 49 b, 49 c areprovided in correspondence with the load circuits 54 a, 54 b, 54 c. Thedetermining circuit 51 determines whether each external switch 30 a, 30b, 30 c (external switch circuit) is switched on or switched off. Theexternal switches 30 a, 30 b, 30 c are used to select whether to supplyor stop supplying the current to the load circuits 54 a, 54 b, 54 c,respectively. The determining circuit 51 selectively activates andinactivates the corresponding first, second, and third FET 49 a, 49 b,49 c, according to the determination.

Therefore, the determining circuit 51 determines whether an intermittentshort circuit has occurred in each load circuit 294 a, 54 b, 54 cseparately. Thus, the determining circuit 51 stops the current supply toonly the load circuit, in which the intermittent short circuit occurs.The flow of current to the load circuits 54 a, 54 b, 54 c is permittedor stopped in accordance with the operation of the external switches 30a, 30 b, 30 c, respectively.

It should be apparent to those skilled in the art that the presentinvention may be embodied in many other specific forms without departingfrom the spirit or scope of the invention. Particularly, it should beunderstood that the invention may be embodied in the following forms.

FIGS. 10A, 10B, 11A, 11B illustrate other embodiments of theintermittent short circuit determining device 90 of the secondembodiment.

Each intermittent short circuit determining device 90A, 90B, 90C, 90Dhas substantially the same circuit as in FIG. 7. The conductive terminal11 is provided in FIGS. 10A, 10B, 11A, 11B instead of the conductiveterminal pattern 42 a.

The intermittent short circuit determining device 90A in FIG. 10A isconnected to a circuit substrate in an electric connection box. Thefirst, second, and third conductive terminals 50 a, 50 b, 50 c, thefirst, second, and third signal input terminals 52 a, 52 b, 52 c, andGND connecting terminal 55 are bent in an L-shaped manner and projectfrom the same side of a molded package 53.

In the intermittent short circuit determining device 90C of FIG. 11A,the first, second, and third conductive terminals 50 a, 50 b, 50 c, thefirst, second, and third signal input terminals 52 a, 52 b, 52 c, andthe GND connecting terminal 55 are bent vertically (downward in FIG.11A) and project from the same side of the molded package 53.

In the intermittent short circuit determining device 90B of FIG. 10B,the first, second, and third conductive terminals 50 a, 50 b, 50 c, thefirst, second, and third signal input terminals 52 a, 52 b, 52 c, theGND connecting terminal 55, and the conductive terminal 11 projectstraightly from the same side of the molded package 53.

In the intermittent short circuit determining device 90D of FIG. 11B,only the conductive terminal 11 projects straightly from the oppositeside of the molded package 53, from which the other terminals project.

In the embodiment shown in FIGS. 12 and 13, the configuration of theelectric circuit differs from that of the intermittent short circuitdetermining device 90 of the second embodiment. The differences from theelectric circuit of the second embodiment will mainly be discussedbelow.

The intermittent short circuit determining device 90E of FIG. 12includes a single FET (relay circuit or disconnecting means) 49, whichis connected to the battery BT. Three branching circuits, which areconnected to the FET 49, are connected to the first, second, and thirdcurrent sensors 45 a, 45 b, 45 c, respectively.

The first, second, and third current sensors 45 a, 45 b, 45 c areprovided with the determining circuits 51 a, 51 b, 51 c, respectively.

Each determining circuit 51 a, 51 b, 51 c receives the switch ON signalor the switch OFF signal from the corresponding external switch 30 a, 30b, 30 c. The external switches 30 a, 30 b, 30 c are used to selectivelyclose and open the circuit including the load circuits 54 a, 54 b, 54 c.Each determining circuit 51 a, 51 b, 51 c selectively activates anddeactivates the FET 49 according to the switch ON/OFF signal. Eachdetermining circuit 51 a, 51 b, 51 c determines whether an intermittentshort circuit has occurred according to the detected voltage (loadcurrent) assorts the two terminals of the corresponding current sensor45 a, 45 b, 45 c. When it is determined that an intermittent shortcircuit has occurred, the associated determining circuit 51 a, 51 b, 51c inactivates the FET 49.

The intermittent short circuit determining device 90E is integrated inthe molded package as in the second embodiment. In the intermittentshort circuit determining device 90E, the load circuits 54 a, 54 b, and54 c are selectively opened and closed by the single FET 49.

The intermittent short circuit determining device 90F in FIG. 13includes the circuit configuration, which is equivalent to that of theintermittent short circuit determining device 90 of the secondembodiment. In addition, the determining device 90F includes separatedetermining circuits 51 a, 51 b, 51 c for detecting the voltage on twoterminals of each first, second, and third current sensor 45 a, 45 b, 45c.

Each determining circuit 51 a, 51 b, 51 c receives the switch ON signalor the switch OFF signal from the corresponding external switches 30 a,30 b, 30 c. The external switches 30 a, 30 b, 30 c are used toselectively close and open the load circuits 54 a, 54 b, 54 c based onthe switch ON/OFF signals. Each determining circuit 51 a, 51 b, 51 cselectively activates and inactivates the FET 49 a, 49 b, 49 cseparately according to the switch ON/OFF signal. Each determiningcircuit 51 a, 51 b, 51 c determines whether an intermittent shortcircuit has occurred according to the detected voltage (load current) ontwo terminals of the corresponding first, second, and third currentsensor 45 a, 45 b, 45 c. When it is determined that an intermittentshort circuit has occurred, each determining circuit 51 a, 51 b, 51 cinactivates the FET 49 a, 49 b, 49 c separately. The intermittent shortcircuit determining device 90F is integrated in the molded package as inthe second embodiment.

The circuit of the intermittent short circuit determining device 90G ofFIG. 14 includes three circuits of the intermittent short circuitdetermining device 90, which are parallel. The intermittent shortcircuit determining device 90G independently determines the occurrenceof the intermittent short circuit in each load circuit 294 a, 54 b, 54 cand performs the close/open control of the load current. One of externalbatteries Ba, Bb, Bc is connected to each independent circuit in theintermittent short circuit determining device 90G.

The current sensor 2 (45 a, 45 b, 45 c) may be an element or circuitthat generates a detection signal that is proportional to the loadcurrent. For example, a sensor that uses a shunt resistor, a thermistor,or a Hall element may be used.

The determining circuits 16 may be set such that the circuits 16determine that an intermittent short circuit has occurred in detectedcurrent IL if the resistance loss in the current sensor 2 is greaterthan the predetermined resistance loss.

The determining circuit 16 may also be set such that the circuits 16determine that an intermittent short circuit has occurred in the loadcurrent if the temperature increase caused by the resistance loss incurrent sensor 2 is greater than the predetermined value.

The determining circuits 16 may be predetermined logic circuits.

The relay circuit may be any circuit, which stops the load currentaccording to the control signal. For example, the relay circuit may be arelay using a semiconductor, or an electromagnetic relay.

The intermittent short circuit determining device of the presentinvention may be embodied in other than the automobile intermittentshort circuit determining device.

Therefore, the present examples and embodiments are to be considered asillustrative and not restrictive and the invention is not to be limitedto the details given herein, but may be modified within the scope andequivalence of the appended claims.

1. An intermittent short circuit determining device located between apower source and a load circuit in a vehicle electric circuit, theintermittent short circuit determining device compromising: means fordetecting current flowing through the load circuit; means fordetermining whether the current detected by the detecting means is anintermittent short circuit; means for disconnecting the load circuitwhen the determining means determines that there is an intermittentshort circuit; wherein the determining means is selectively connected toan external switch circuit, which supplies and stops current to the loadcircuit, and wherein the determining means determines whether theexternal switch circuit is switched on or off, and switches thedisconnecting means on or off according to said determination.
 2. Theintermittent short circuit determining device according to claim 1,wherein a fuse is included in the detecting means.
 3. The intermittentshort circuit determining device according to claim 1, wherein asemi-conductor relay is included in the disconnecting means.
 4. Theintermittent short circuit determining device according to claim 1,wherein the detecting means, the determining means, and thedisconnecting means are integrally packaged.
 5. The intermittent shortcircuit determining device according to claim 1, wherein the detectingmeans is one of a plurality of detecting means, and each of thedetecting means corresponds to a load circuit from a plurality of loadcircuits, respectively, wherein the disconnecting means is one of aplurality of disconnecting means, and each of the disconnecting meanscorresponds to a load circuit from said plurality of load circuits,respectively, wherein the determining means is connected to theplurality of detecting means and the plurality of disconnecting means,and the determining means is connected to a plurality of external switchcircuits corresponding to the plurality of load circuits, respectively,and each external switch circuit selectively supplies and stops currentto the load circuit, and wherein the determining means determineswhether the plurality of external switch circuits are switched on oroff, and selectively switches the disconnecting means on or offaccording to said determination.
 6. The intermittent short circuitdetermining device according to claim 1, wherein the detecting means isone of a plurality of detecting means, and each of the detecting meanscorresponds to a load circuit from a plurality of load circuits,respectively, wherein the disconnecting means is connected between apower source and the plurality of detecting means, and the disconnectingmeans is able to stop load current supplied to all of the load circuits,and wherein the determining means is one of a plurality of determiningmeans, and each determining means is selectively connected to one of aplurality of external switch circuits, and the plurality of externalswitch circuits corresponds to the plurality of load circuits,respectively, and each external switch circuit selectively supplies andstops current to the load circuit, and wherein each determining meansdetermines whether the external switch circuit is switched on or off,and selectively switches the disconnecting means on or off according tosaid determination.
 7. The intermittent short circuit determining deviceaccording to claim 1, wherein the detecting means is one of a pluralityof detecting means, and each of the detecting means corresponds to aload circuit from a plurality of load circuits, respectively, whereinthe disconnecting means is one of a plurality of disconnecting means,and each of the plurality of disconnecting means corresponds to a loadcircuit from the plurality of load circuits, respectively, and whereinthe determining means is one of a plurality of determining means, andeach determining means is selectively connected to one of a plurality ofexternal switch circuits, and each external switch circuit from theplurality of external switch circuits corresponds to a load circuit fromthe plurality of load circuits, respectively, and each external switchcircuit selectively supplies and stops current to the load circuit, andwherein each determining means determines whether the external switchcircuit is switched on or off, and switches the disconnecting means onor off according to the determination.
 8. The intermittent short circuitdetermining device according to claim 7, wherein each of the pluralityof detecting means is connected to a corresponding separate powersource, and each of the plurality of disconnecting means is able to stopthe load current supplied from the corresponding power source.
 9. Anintermittent short circuit determining device for determining whether anintermittent short circuit has occurred in a load circuit, theintermittent short circuit determining device comprising: means fordetecting a load current flowing through the load circuit and forgenerating a detection signal; determining means connected to thedetecting means, wherein the determining means determines whether anintermittent short circuit has occurred based on the detection signal;and disconnecting means connected to the determining means, wherein thedisconnecting means stops supply of the load current to the load circuitwhen it is determined that an intermittent short circuit has occurred,wherein the determining means determines whether an intermittent shortcircuit has occurred based on at least one of four characteristicquantities, which include the load current flowing through the loadcircuit, a time period during which the load current exceeds apredetermined current threshold value, an ON-duty ratio indicating theratio of a time period when the load current exceeds the currentthreshold value relative to a predetermined time period, and an actualover-current number indicating the number of times that the load currentexceeds the current threshold value during a predetermined time period,and wherein the determining means switches the disconnecting means on oroff according to said determination.
 10. The intermittent short circuitdetermining device according to claim 9, wherein the detecting means,the determining means, and the disconnecting means are integrallypackaged.
 11. The intermittent short circuit determining deviceaccording to claim 9, wherein when at least two of the fourcharacteristic quantities exceed predetermined reference values, thedetermining means determines that an intermittent short circuit hasoccurred.
 12. The intermittent short circuit determining deviceaccording to claim 11, wherein the detecting means, the determiningmeans, and the disconnecting means are integrally packaged.
 13. Theintermittent short circuit determining device according to claim 9,wherein the determining means includes a processing circuit whichgenerates a current flow permission signal and a current shutdown signalwherein the current flow permission signal activates the disconnectingmeans and allows the load current to flow, and the current shutdownsignal inactivates the disconnecting means and stops the load current.14. The intermittent short circuit determining device according to claim9, wherein the current threshold value is one of a plurality of currentthreshold values, and the plurality of current threshold values definesa plurality of load current ranges, and each of the plurality of loadcurrent ranges is associated with at least one of a reference ON-dutyratio and a reference over-current number, and the determining meanscompares the load current with the plurality of current threshold valuesand identifies a load current range wherein the load current belongs,and when at least one of the ON-duty ratio and the actual over-currentnumber exceeds the corresponding reference value associated with theidentified load current range, the determining means determines that anintermittent short circuit has occurred.
 15. The intermittent shortcircuit determining device according to claim 14, wherein thedetermining means includes a storage device for storing the plurality ofcurrent threshold values and at lease one of the reference ON-duty ratioand the reference over-current number.
 16. A method for determining theoccurrence of an intermittent short circuit in a load circuit, whereinthe method includes performing at least one of four comparing stepsincluding a current comparison step, a time period comparison step, aduty ratio comparison step, and an over-current number comparison step,wherein the current comparison step comprises comparing the value ofload current flowing through the load circuit with a current thresholdvalue, the time period comparing step comprises comparing a time periodduring which the load current exceeds the current threshold value with areference time period, the duty ratio comparison step comprisescomparing an ON-duty ratio with a reference ON-duty ration, where theON-duty ratio indicates the ratio of a time period during which the loadcurrent exceeds the current threshold value relative to a predeterminedtime period, and the over-current number comparison step comprisescomparing an actual over-current number indicating the number of thetimes that the load current exceeds the current threshold value duringthe predetermined time period with a reference over-current number. 17.The determining method according to claim 16, wherein the currentthreshold value is one of a plurality of current threshold values, andthe plurality of current threshold values defines a plurality of loadcurrent ranges, and each of the plurality of load current ranges isassociated with at least one of the reference ON-duty ratio and thereference over-current number, wherein the current comparing stepincludes comparing the load current with the plurality of currentthreshold values and determining a load current range to which the loadcurrent belongs, and the determining method further includes determiningthat an intermittent short circuit has occurred when at least one of theON-duty ratio and the actual over-current number exceeds thecorresponding reference value associated with the determined loadcurrent range.
 18. A method for determining the occurrence of anintermittent short circuit in a load circuit, wherein the methodincludes at least two of four comparing steps including a currentcomparison step, a time period comparison step, a duty ratio comparisonstep, and an overcurrent number comparison step, wherein the currentcomparison step comprises comparing the value of a load current flowingthrough the load circuit with a current threshold value, the time periodcomparing step comprises comparing a time period during which the loadcurrent exceeds the current threshold value with a reference timeperiod, the duty ratio comparison step comprises comparing an ON-dutyratio with a reference ON-duty ratio, and the ON-duty ratio indicatesthe ratio of a time period during which the load current exceeds thecurrent threshold value relative to a predetermined time period, and theover-current number comparison step comprises comparing an actualover-current number indicating the number of times that the load currentexceeds the current threshold value during the predetermined time periodwith a reference over-current number.
 19. The determining methodaccording to claim 18, wherein the current threshold value is one of aplurality of current threshold values, and the plurality of currentthreshold values define a plurality of load current ranges, and each ofthe plurality of load current ranges is associated with at least one ofthe reference ON-duty ratio and the reference over-current number,wherein the current comparing step includes comparing the load currentwith the plurality of current threshold values and determining a loadcurrent range to which the load current belongs, and the determiningmethod further includes determining that an intermittent short circuithas occurred when at least one of the ON-duty ratio and the actualovercurrent number exceeds the corresponding reference value associatedwith the determined load current range.
 20. A method for determiningwhether an intermittent short circuit has occurred in a load circuit,the method compromising: comparing a load current that flows in the loadcircuit with a first current threshold value; comparing the load currentwith a second current threshold value, the second current thresholdvalue being greater than the first current threshold value; comparing afirst time period, during which the load current exceeds the firstcurrent threshold value, with a first reference time period; comparing asecond time period, during which the load current exceeds the secondcurrent threshold value, with a second reference time period; comparingan ON-duty ratio with a reference ON-duty ratio wherein the on-dutyratio is the ratio of a time period when the load current exceeds thesecond current threshold value relative to a predetermined time period;and comparing an overcurrent number with a reference overcurrent number,wherein the overcurrent number is the number of times that the loadcurrent exceeds the second current threshold value during apredetermined time period.
 21. The determining method according to claim20, wherein the method further comprises: determining that when the loadcurrent exceeds the first current threshold value and when the firsttime period exceeds the first reference time period, an intermittentshort circuit has occurred; determining that when the load currentexceeds the second current threshold value and when the second timeperiod exceeds the second reference time period, an intermittent shortcircuit has occurred; determining that when the load current exceeds thesecond current threshold value and when the ON-duty ratio exceeds thereference on-duty ratio an intermittent short circuit has occurred; anddetermining that when the load current exceeds the second currentthreshold value and when the actual over-current number exceeds thereference over-current number, and intermittent short circuit hasoccurred.